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AleroB888 05-27-2016 06:07 PM

New Engine, Misfires at Low RPM
Part I

This is a fresh rebuilt engine with porting of intake and heads, beefed up bottom end but a not too radical cam.

There are pretty severe misfires at lower RPMs, and vibration, but gets considerably smoother higher up in the RPMs. Top end torque is good, much better than stock. The misfires appear real, given the rough running of the engine, but the plugs look perfectly clean when checked. Exhaust vapor is clean. There is only about 200 miles on the engine so far.

In particular, this is a 3400 built-up block, 3500 heads, 3500 LIM, 3400 UIM. '99 timing chain, CR appox. 9.1, Cam is a step up from stock.

I had a local shop install the engine. When it was first fired up, they said it ran about 10 seconds then blew oil out the oil filter gasket. They put another filter on and it happened again. So I told them to replace the oil pump, which I still have. This time it stayed running, but only if the gas pedal was held slightly open. I drove it 3 miles home, but it ran very rough, a lot of vibration.

Originally, there was synthetic oil put in, but I drained it out after about 20 minutes of run time, 3 miles of driving, then changed to Autozone brand regular oil. The first oil was darker, but no visible particles. Second oil change at 50 miles, looked perfectly clean.

After some idling and low speed driving, the misfires remained. The power/ torque seems to be excellent, it does better cruising when above 2000 RPM.
I want to believe that there is some kind of ignition or electrical issue, but I need to hear all possible theories on it. It idles like an old Harley lol....

AleroB888 05-27-2016 06:12 PM

Part II

I have now replaced these ignition-related part in the system:

Cam sensor,
different coils and ignition module (whole assemblies),
new plug wires,
different crank sensor (rear of engine block),
MSD DIS unit has been removed to simplify things.
Crank pulley and sensor
Upper intake gaskets replaced with thicker ones to make sure manifold clears fuel rails.
2 different sets of injectors.

Oil changes were done again at about 65 and 400 for a total of 3.

I have put a little over 600 miles on it.

During the first 200 miles, a line from the PCV valve got plugged, forcing some vapors out the back valve cover tube. But away from the PCV lines, oil seeps down along the edges of the front head gasket and drips from the corners. It has not been nearly as bad since the PCV line was cleared, but there is still some dripping down both edges of the gasket. It appears to happen only when the engine runs, and does not drastically affect the oil level after a couple hundred miles. The leak appears to be from the LIM/block seam, near the throttle body

So I still have the engine vibration and the misfires when idling or cruising at low speed below about 1500 RPM. The engine starts well enough cold and continues to run, as long as it can warm up for a few minutes. But backing the car out of the garage just after starting can cause it to die.

When fully hot and on the road it does better at high RPM, but it has trouble maintaining idle at stoplights, and surges cruising at low speed. When it is fully hot and then shut off and parked for about a half hour, restarting is very difficult, even if resorting to opening the throttle.

I put a stock front trans mount back on, but the engine itself moves around more, even though less vibrations get to the chassis.

Sets the P300 misfire code. When deleted, it comes back every other startup. The misfires occur on all cylinders, but are more numerous on cyl 6, then cyl 5 is a close second. Misfires disappear around 1500-2000 when cruising, and are not showing up cruising at over 50 mph or so.

Delivered Trans Torque scans 45 ft lbs more than a bone stock vehicle. Has decent power off the line. (still normally aspirated)

I don't think synthetic oil being in there at first start caused this, since it was changed after only 3 miles and 20 minutes of driving. The excess oil pressure at first startup I'm worried about, though, and I wonder if that caused the oil leak at the LIM/block junction.

Need some informed speculation on possible causes...

Thanks for reading (whew).

O1GAGT 05-27-2016 06:43 PM

Have you tried a different pcm with a full write?
Are the injectors set correctly in the pcm?
Has timing been adjusted in the pcm or are you running the stock spark tables?

Have you ran a compression test on the engine?
What plugs and what wires?

If I remember correctly some of the 3800 guys got the P300 code and had to turn off reporting of the code when running an aftermarket cam.

O1GAGT 05-27-2016 07:07 PM

Also what transmission and torque converter? Stock?

AleroB888 05-27-2016 07:38 PM


Originally Posted by O1GAGT (Post 1265612)
Have you tried a different pcm with a full write?
Are the injectors set correctly in the pcm?
Has timing been adjusted in the pcm or are you running the stock spark tables?

Have you ran a compression test on the engine?
What plugs and what wires?

If I remember correctly some of the 3800 guys got the P300 code and had to turn off reporting of the code when running an aftermarket cam.

Also what transmission and torque converter? Stock?

No, have not tried a different PCM, I should be able to do that.

Injectors set for 35/36 lb. I took the original engine back to stock (blower removed) and changed the PCM tables where needed, before they did the swap.

I had to disable some DTCs including the misfire code in order to do the crank relearn, which I did twice after each crank sensor change.

Transmission and converter are upgraded, been in use for 3 years, but I think the stall speed is not too high.

I have not done a leakdown test, or a compression test. It's been years since I did a compression test on the original engine, and I don't have much experience in the methodology. Before it goes back in the garage, though, I want to gather as much info as I can. Been busy on other projects as well :)

mfuller 05-27-2016 07:43 PM

I know it seems trivial, but are you sure that your spark plug wires are installed correctly? Also is it possible that your pushrods are a touch too long?

plastic_indian 05-29-2016 11:04 PM

Verify the oil pressure using a known good manual gauge. Is this build running a HV and/or HP oil pump? In an extreme case, perhaps excessive oil pressure to hydraulic tappets is floating the valves...

Is the cam a custom grind (is it ground 'straight up' or with a built-in advance?)? Are the cam/crank gears adjustable? Was the cam degreed during the build to verify its position? Was valve installed height, coil bind, general valve train geometry verified?

A cold/warm compression test may provide some information. Disable fuel and spark, then test with the throttle wide open.

Testing crankcase pressure may shed some light here as well.

rocketfast123 06-01-2016 12:10 PM

From the backyard tech. Any leaks around the mass air flow? Friend had a ranger do that and the intake pipe after the mass had a hole. Tsp also could be on it way out. Also coolant temp , and iat.

AaronGTR 06-04-2016 10:02 PM

I'm with Mark. Start out with the basic tests first. Cranking compression and leak down test should be the first thing you do to tell the cylinders and heads are in good condition. IE rings are good/proper gap, valves are sealing properly, gaskets aren't leaking, etc. Any one of those could cause misfires. Then look at possible valve train issues, ie cam profile is correct, lifters are working properly, cam timed properly, etc.

I would also verify the oil pressure with a good mechanical gauge, and make sure you're using the correct pump. Should NOT be using a HV pump on a non-racing engine without proper clearances. The fact it was blowing out gaskets tells me one of two things. Either the oil pressure is wrong, or there's something upstream blocking flow and/or the pressure bypass isn't working properly. If it was, and the pressure's correct, there's no reason it should ever blow the filter gasket.

Also, there's no reason to do multiple short oil changes. One oil change after 500-1000 miles of mixed load driving should be all that's needed. A properly built modern engine does not wear in very much, so there is nothing in the oil to get rid of. Basically only the rings should wear in and seat in the cylinders, and everything else should be machined to proper clearances so they are separated by a film of oil and never touch. If they do, you have major problems.

3400cavyhatch 07-04-2016 04:26 PM

Did you get this problem figured out?

AleroB888 07-05-2016 03:14 PM


Originally Posted by 3400cavyhatch (Post 1265720)
Did you get this problem figured out?

No, still working on other projects. I ran it today about 45 miles, though.

The oil pump is a Melling M134...(not HV)

Slinky 07-06-2017 06:52 AM

Any updates?
Its been awhile, im excited to see what becomes of this build.

O1GAGT 07-06-2017 07:16 PM

I had a call from Greg a few weeks ago, letting me know that he had been having some health issues. He indicated that he wanted to give me a heads up to see if I may have any interest in the white Alero as he hasn't been able to mess with it. We didn't really discuss pricing much and I haven't heard anything more from him since.

AleroB888 09-11-2017 02:39 PM

^Thanks, Phil.

Yeah, I'll be trying to get things organized to sell the white Alero (1999).

The new engine still runs smoothly above about 1600 rpm, but idles rough with random misfires as previously discussed.

I want to put the MP90 back on, and sell it complete that way if possible.

Now, since the engine runs well at upper rpm, and had some time to break-in, about 1K miles, will I be able to gain any insight on the engine's idle problem if some boost is applied?

I don't intend to run it hard at all, of course, and at low mph. But I do want whoever test drives it to get an idea what it's capable of....

Slinky 09-12-2017 09:39 AM

I wish I had some info to lend you that would help, the only thing I know is that when my car was tuned the tuner voided the knock sensor at low rpm due to it causing misfires from my cam, and my cam is rather mild coupled with my supercharger.

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